The present invention relates to a transparent shrinkable film comprising a base layer containing a mixture of polypropylene and hydrocarbon resin. The invention also relates to a process for manufacturing such a film and to the use of the film as shrink labels.
It is known from "Verpackungsrundschau" (Packaging Review) No. 10/1983, pp. 1121-1122, to surround cylindrical or substantially cylindrical packages, such as bottles or cans with overall labels; these labels, which entirely surround the circumferential surface of the packages are also known as sleeves. The films used for this purpose are, for example, made from polyethylene, polybutylene, polystyrene, copolymers of polyethylene and polypropylene, copolymers of ethylene and vinyl acetate or various blends, but in particular from polyvinyl chloride. To achieve the desired shrink properties in the circumferential direction of the sleeves, necessary for the intended use, the films usually are oriented by biaxial stretching. This may, for example, be performed using a bubble, stenter or calendar process, paying particular attention to the orientation in the transverse direction. To accomplish an absolutely crease-free, tight contact between the sleeve and the package, the shrink values measured after a treatment for about 15 minutes at 90.degree. C. in a circulating air cabinet should be about 20 to 40% in the transverse direction (s.sub.t) and not more than 6 to 8% in the longitudinal direction (s.sub.l).
The sleeves are provided with prints, for example, by reverse side printing, and then converted into a tubing by gluing or welding. Due to the controlled transverse shrink, a tight, crease-free contact is created between the tube and the package in the shrink tunnel. The sleeves are applied automatically with the aid of brushes or manually. In addition to the desired shrink the sleeves must process the following properties: high gloss, clarity, good slip and stability (corresponding to the product of modulus of elasticity and thickness.sup.3) for the automatic application of the sleeve, good printability and good welding/bonding characteristics.
Among the known films, PVC films are the films which, in accordance with U.S. Pat. No. 4,352,849, best fulfill the demands set forth above. Their high shrink capacities make them suitable for almost all applications. The shape or diameter of a package, in particular of a can or bottle, can vary up to 30% or even more in the surface area to which the sleeve is to be applied.
A particular disadvantage of PVS sleeves is their high price. The high price mainly results from the high density of 1.39 kg/dm.sup.3, which is about 50% higher than the density of, for example, polypropylene. Furthermore, problems are encountered with regard to corrosion of the manufacturing and processing apparatus (see EP-A-O 233 400).
Polyolefinic films for shrink labels predominantly comprise blends of homo-, co- and terpolymers. To achieve the required shrink properties, the films are biaxially stretched by means of a bubble or stenter process.
Polylefinic shrink labels known from the art possess properties which may be undesirable during their processing for practical use, i.e., in most cases, their mechanical properties are poor and/or the process for accomplishing high transverse and low longitudinal shrinkage is very expensive and complicated.
Insufficient mechanical strength values are of particular disadvantage when automatic shrink appliances are used. To achieve satisfactory mechanical strength values, i.e., values comparable to those of PVC films, the thickness of polyolefinic film in some cases has to be increased by up to 50% or even more. This leads to correspondingly higher costs. Moreover, the shrink rate is slowed by the increased film thickness, so that it takes more time to achieve a predetermined final shrink value. A state-of-the-art process for achieving a high transverse shrink/longitudinal shrink ratio of polypropylene films is, for example, described in EP-A-O 171 733. This publication also discloses films of the type described at the outset, comprising copolymers of propylene and other alphaolefins and a resin admixture. According to this publication, a biaxially oriented film is produced in a two-stage process. To achieve a low shrinkage in the longitudinal direction, the film is subjected to an additional tempering treatment at an elevated temperature (about 130.degree. C.) between the longitudinal stretching and the transverse stretching. This heat treatment is performed for a period of between 2 and 180 seconds. Depending on the advance speed of the film in the production line--usually 200 to 300 m/min--the process requires the installation of an expensive and complicated addition between the sections for longitudinal and transverse stretching, for at a tempering period of, for example, 60 seconds and a machine speed of 200 m/min, the oven would have to be 200 m long. Thus, the method is not suited for modern film production processes from the standpoint of process engineering. Moreover, the mechanical properties of the film are still not satisfactory.